Can Afforestation Make Cities Cooler? The Urban Heat Island Effect Explained

By Ketul

Updated 07 March, 2025

10 min read

What is The Urban Heat Island Effect?

Structures such as buildings, roads, and other infrastructure absorb and re-emit the sun’s heat more than natural landscapes such as forests and water bodies. Urban areas, where these structures are highly concentrated and greenery is limited, become “islands” of higher temperatures relative to outlying areas. These pockets of heat are referred to as “heat islands.” Heat islands can form under a variety of conditions, including during the day or night, in small or large cities, in suburban areas, in northern or southern climates, and in any season. Heat islands are usually measured by the temperature difference between cities relative to the surrounding areas.

Impacts of Heat Islands

Increased energy consumption – Heat islands increase electricity demand for air conditioning and peak energy demand on exceptionally hot afternoons, when offices and homes are running air-conditioning systems, lights, and appliance.
Elevated Emissions of Air Pollutants and Greenhouse Gases – To meet electricity needs, utility companies typically rely on fossil fuel power plants as a power source. Increased use of fossil fuels increases emissions of greenhouse gases, such as carbon dioxide, which contribute to global climate change.
Compromised Human Health and Comfort – Hot weather events contribute to heat-related deaths and heat-related illnesses
Impaired Water Quality – High temperatures of pavement and rooftop surfaces can heat stormwater runoff, which drains into storm sewers. This runoff raises water temperatures as it is released into streams, rivers, ponds, and lakes. Water temperature affects the metabolism and reproduction of many aquatic species.

Causes of Heat Islands

Heat islands form as a result of several factors:

Reduced Natural Landscapes in Urban Areas

Urban Material Properties – Conventional human-made materials used in urban environments that reflect less solar energy, and absorb and emit more of the sun’s heat

Urban Geometry – In heavily developed areas, surfaces and structures obstructed by neighboring buildings become large thermal masses that cannot release their heat readily. Cities with many narrow streets and tall buildings become urban canyons, which can block natural wind flow that would bring cooling effects

Heat Generated from Human Activities such as vehicles, air-conditioning units, buildings, industrial facilities

Why do we need green areas in our cities?

Nature breathes life into our cities – from green spaces such as urban gardens to blue spaces such as ponds and lakes full of intricate ecosystems. Cultivating and caring for urban nature creates opportunities for healthy and sustainable livelihoods especially as cities increase their resilience to a changing climate. This includes improving our physical and mental health and having access to better employment and economic outcomes. Data shows equitable access to nature provides significant and measurable benefits to both people and the environment, protects public health in the wake of the pandemic, and builds more inclusive economies. Nature in our cities serves as a natural buffer and regulator of climate impacts and protects urban residents and city infrastructure from extreme heat, flooding, drought, sea level rise and storms. Designing and deploying natural solutions that create an equilibrium with changing climatic conditions will be critical to our cities’ resilience and our planet’s future.

Exactly how do trees and vegetation reduce heat?

Trees and vegetation (e.g., bushes, shrubs, tall grasses) lower surface and air temperatures by providing shade and cooling through evapotranspiration. Evapotranspiration is a process in which trees and vegetation absorb water through their roots and evaporate it through their leaves. Essentially, evapotranspiration cools the air by using heat from the air to evaporate water. This cooling also occurs from the surrounding soil and when trees and vegetation catch rainfall on their leaves. On average, urban forests are 3.0° F (1.6°C) cooler than urban non-green areas.

Co-Benefits of Heat Island Mitigation Strategies

Heat islands form as a result of several factors:

Reduce need for artificial cooling – Trees and vegetation that directly shade buildings decrease demand for air conditioning. This in turn reduces emissions of greenhouse gases from fossil fuel energy production, helping to mitigate climate change.

Reduce Pollution – Airborne pollutants may deposit on tree leaves, directly removing them from the air. These include particulate matter (PM), nitrogen oxides (NOX), sulfur dioxide (SO2), carbon monoxide (CO), and ground-level ozone (O3). Roadside vegetation that is tall and dense can lessen downwind pollutants by approximately 30%.

Water Quality – Trees and vegetation support water quality in three ways. First, they absorb excess rainfall, thus reducing the amount of water that runs off roofs and streets into local water bodies. Second, trees and vegetation filter fertilizers, pesticides, animal waste, and other pollutants, which readily runoff paved surfaces during storms. Third, trees provide shade that can cool water runoff and prevent water temperature shocks to aquatic life.

Human Health – Studies show that human interaction with nature reduces high heart rate and blood pressure, and increases immune system function. Additionally, trees and vegetation can act as a physical barrier to reduce noise and light pollution.

Wildlife Biodiversity – Trees and vegetation provide essential habitat and food for many species, including pollinating insects, migratory birds, and small mammals (e.g., bats, squirrels).

How does Medellin do it?

With “green corridors” that mimic the natural forest, Medellín is driving down temperatures — and could become five degrees cooler over the next few decades. In the face of a rapidly heating planet, the City of Eternal Spring — nicknamed so thanks to its year-round temperate climate — has found a way to keep its cool.

Previously, Medellín had undergone years of rapid urban expansion, at the expense of green spaces and vegetation, which led to a severe urban heat island effect — raising temperatures in the city to significantly higher than in the surrounding suburban and rural areas. The city launched a new approach to its urban development — one that focused on people and plants.

The $16.3 million initiative led to the creation of 30 Green Corridors along the city’s roads and waterways, improving or producing more than 70 hectares of green space, which includes 20 kilometers of shaded routes with cycle lanes and pedestrian paths.

These plant and tree-filled spaces — which connect all sorts of green areas such as the curb strips, squares, parks, vertical gardens, sidewalks, and even some of the seven hills that surround the city — produce fresh, cooling air in the face of urban heat. The corridors are also designed to mimic a natural forest with levels of low, medium and high plants, including native and tropical plants, bamboo grasses and palm trees.

Heat-trapping infrastructure like metro stations and bridges has also been greened as part of the project and government buildings have been adorned with green roofs and vertical gardens to beat the heat. At the launch of the project, 120,000 individual plants and 12,500 trees were added to roads and parks across the city. By 2021, the figure had reached 2.5 million plants and 880,000 trees. Each has been carefully chosen to maximize their impact. “The technical team thought a lot about the species used. They selected endemic ones that have a functional use,” explains Paula Zapata, advisor for Medellín at C40 Cities.

The 72 species of plants and trees selected provide food for wildlife, help biodiversity to spread and fight air pollution. A study, for example, identified Mangifera indica as the best among six plant species found in Medellín at absorbing PM2.5 pollution — particulate matter that can cause asthma, bronchitis and heart disease — and surviving in polluted areas due to its “biochemical and biological mechanisms.”

And the urban planting continues to this day.

Medellín’s temperatures fell by 2°C in the first three years of the program, and officials expect a further decrease of 4 to 5C over the next few decades, even taking into account climate change. In turn, City Hall says this will minimize the need for energy-intensive air conditioning. A separate study estimated that in just one of Medellín’s corridors, the new vegetation growth would absorb 160,787 kg of CO2 per year and that over the next century 2,308,505 kg of CO2 will be taken up – roughly the equivalent of taking 500 cars off the road.

In addition, the project has had a significant impact on air pollution. Between 2016 and 2019, the level of PM2.5 fell significantly, and in turn the city’s morbidity rate from acute respiratory infections decreased from 159.8 to 95.3 per 1,000 people.

There’s also been a 34.6 percent rise in cycling in the city, likely due to the new bike paths built for the project, and biodiversity studies show that wildlife is coming back — one sample of five Green Corridors identified 30 different species of butterﬂy.

This raises hope and shows a way for cities around the world to mitigate the effects of climate change as far as heat and pollution are concerned.

Other cities that are doing it right! - Singapore

Singapore is one of the greenest cities in the world. After 6 decades of greening efforts, they have a thriving network of green spaces where nature is entwined into their everyday lives. Currently, there are 50 nature ways in Singapore, stretching 210km in total. As part of the Singapore Green Plan 2030 to transform Singapore into a City in Nature, they aim to have 300km of Nature Ways by 2030. Nature ways are routes planted with specific trees and shrubs that replicate the natural structure of forests. This is to facilitate the movement of animals like birds and butterflies between 2 green spaces and connect areas of rich biodiversity, such as the Western Catchment (SAFTI Live Firing Area), Central Catchment Nature Reserve and Bukit Timah Nature Reserve, to urban communities in order to create a greater appreciation of our City in Nature.

The United Nations’ Trillion Tree Campaign

There is enough room in the world’s existing parks, forests, and abandoned land to plant 1.2 trillion additional trees, which would have the CO2 storage capacity to cancel out a decade of carbon dioxide emissions, according to a new analysis by ecologist Thomas Crowther and colleagues at ETH Zurich, a Swiss university.

The research, presented at this year’s American Association for the Advancement of Science conference in Washington, D.C., argues that planting additional trees is one of the most effective ways to reduce greenhouse gases.

“There’s 400 gigatons [of CO2 stored] now in the 3 trillion trees,” Crowther said. “If you were to scale that up by another trillion trees, that’s in the order of hundreds of gigatons captured from the atmosphere – at least 10 years of anthropogenic emissions completely wiped out.”

Tree planting is becoming an increasingly popular tool to combat climate change. The United Nations’ Trillion Tree Campaign has planted nearly 15 billion trees across the globe in recent years. And Australia has announced a plan to plant a billion more by 2050 as part of its effort to meet the country’s Paris Agreement climate targets.

C40 cities

C40 is a global network of nearly 100 mayors of the world’s leading cities that are united in action to confront the climate crisis. Mayors of C40 cities are committed to using an inclusive, science-based and collaborative approach to cut their fair share of emissions in half by 2030, help the world limit global heating to 1.5°C, and build healthy, equitable and resilient communities.

The C40 Urban Nature Accelerator was launched in 2021 to make cities greener, healthier and more resilient. huge progress has been seen from signatory cities towards the targets of the C40 Urban Nature Accelerator in the last two years. These cities are making major investments to expand nature and increase access for residents.

Their mayors are committing to protect communities from climate risk and help meet the goals of the Paris Agreement while improving overall health and well-being, these cities pledge to increase and enhance nature in their urban environments that reduce climate risk and vulnerability, supports wider ecosystem services, and is equitably distributed and publicly accessible, by 2030.

To achieve this goal, the signatories of the C40 Urban Nature Accelerator will deliver on one or both of the following pathways and support greener and more resilient cities:

The Miyawaki method

In an attempt to increase urban forestation, many cities and counties have also begun adopting the Miyawaki method. It has started a global movement towards reforestation, resulting in the emergence of small urban forests in many parts of the world. It was developed by a famous Japanese ecologist who discovered a way to reconstruct forests using indigenous species more quickly than conventional methods.

The Miyawaki reforestation method aims to recreate indigenous forest ecosystems by planting seedlings of native species in close proximity to each other. This is achieved through careful analysis of the soil and making necessary adjustments. Dozens of locally-sourced plant species are then randomly combined in a dense manner, with up to 20 to 30 times more density than commercial forestry.

A relevant example comes from Lahore in Pakistan. Once called the ‘City of Gardens,’ Lahore has become one of the most polluted places on earth. To counter this, Prime Minister Imran Khan launched a reforestation project in 2021. Dubbed the world’s largest Miyawaki urban forest, its goal is to create a dense forest that will filter pollutants and provide other benefits like carbon sequestration, soil conservation, and biodiversity conservation.

CATCH Foundation

A local Ahmedabad based NGO, CATCH Foundation, is combining the Miyawaki technique with their own modifications and working on reforesting urban areas in India to make them greener and more sustainable. They have planted more than 5,00,000 trees so far in 18 cities across India and created more than 100 multi-species dense forests. To read more about them and their work, click here.

Our future: Nature-based solutions

Nature-based Solutions are defined by the International Union for Conservation of Nature as “actions to protect, sustainably manage, and restore natural or modified ecosystems, that address societal challenges effectively and adaptively, simultaneously providing human well-being and biodiversity benefits”.

“The Green Corridor project is an excellent example of how city planners and governments can use nature for smart urban design”, said Juan Bello, Head of UN Environment in Colombia. Cities like Medellín, Singapore and many others are showing how we can both mitigate and adapt to climate change thanks to nature-based solutions. Governments will need to look hard at deploying such solutions if the world is serious about meeting the goals of the Paris Agreement. It’s time for everyone else to join the coolest movement for the planet and make their own difference!

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